The increased use of coordination of the tripping sequences of circuit breakers in a system has placed additional requirements on each individual circuit breaker. To allow the downstream circuit breaker the necessary time to interrupt a fault, the upstream circuit breaker must be able to withstand higher fault currents.
Thermal magnetic circuit breakers of the prior art do not have the capability of withstanding fault currents because their trip levels are dependent upon the magnetic, thermal and other physical characteristics of their components which react to the current level and duration of the overcurrent. Tripping in these circuit breakers is a function of the physical components. The length of time before tripping occurs cannot be easily adapted to meet withstand requirements mandated by a specific system application. With the introduction of electronic circuit breakers, circuit breakers with withstand capability became technically feasible.
A circuit breaker with withstand capability often utilizes the opposing current flow in the line terminal 30 and lower blade 26, as shown in FIG. 5, to create a blow-on electromagnetic force. This blow-on electromagnetic force causes the lower contacts to move upwards against the moving contacts. This force increases the contact pressure and opposes the constriction force which tends to force the contacts apart. Since both the blow-on electromagnetic force and the constriction force increase with the square of the current through the circuit breaker, the blow-on loop is designed so that the blow-on electromagnetic force is always greater than the constriction force, enabling the circuit breaker to withstand required fault level.
As the blow-on electromagnetic force increases, the circuit braker operating mechanism which is latched to hold the contacts closed, must withstand increasing levels of force. When the fault current exceeds the withstand level, it is desirable to quickly trip the circuit breaker to protect the operating mechanism.
Although electronic components are easily adaptable to withstand varying requirements and other time delays, they are relatively slow to sense a fault. Electronic circuit breakers will signal the circuit breaker to trip approximately ten to fifteen milliseconds after sensing a fault. Because of the high current level of withstand requirements of circuit breakers, it is undesirable to postpone the interruption of current for the length of the time signal delay required by the electronic components. The fault interruption requirements of a given circuit breaker generally are much larger than the withstand level requirements of the same circuit breaker. For example, the circuit breaker described herein has a withstand capability of 35,000 amperes and a fault interrupting capability of 200,000 amperes. After the withstand level is exceeded, the current may quickly rise to the maximum fault level, creating excessive forces that may, on occasion damage the circuit breaker components, especially the operating mechanism.
An electromechanical method is thus desirable for tripping the circuit breaker when the fault has exceeded the time delay allowed for the withstand level. Prior art circuit breakers use a high magnetic yoke and armature to trip the latch of the circuit breaker mechanism. That type of device, if malfunctioning or improperly adjusted, may damage the circuit breaker operating mechanisms as current and forces associated with a contact blow-on loop increase.
There is a need for a circuit breaker that utilizes the forces created by the blow-on loop to trip the circuit breaker immediately upon the circuit breaker current exceeding the time and current characteristics of the withstand level requirement.
There is a need for a circuit breaker that quickly interrupts the current after the fault level has surpassed the required withstand level.
These and other features of the invention will become more readily apparent from the following description, claims and drawings.